In cases where uninvolved bystanders pay to punish defectors, this behavior has typically been interpreted in terms of group-level rather than individual-level benefits. Male cleaner fish, Labroides dimidiatus, punish their female partner if she cheats while inspecting model clients. Punishment promotes female cooperation and thereby yields direct foraging benefits to the male. Thus, third-party punishment can evolve via self-serving tendencies in a nonhuman species, and this finding may shed light on the evolutionary dynamics of more complex behavior in other animal species, including humans.